Process for forming a refractory coating on silicon-iron stock



United States Patent 3,389,006 PROCESS FQR FORMING A REFRACTORY COATEN G 0N SILICON JRON STOCK Dale M. Kohler, Middletown, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio No Drawing. Filed May 18, 1964, Ser. No. 368,422

7 Claims. (Cl. 117-33) ABSTRACT OF THE DISCLOSURE A process for forming a refractory coating on siliconiron sheet stock prior to the final annealing operation producing cube-on-face orientation, comprising the steps of applying a thin binder layer in liquid form to at least one side of the stock, passing the strip and binder layer into contact with fine particles of a refractory separator substance causing the particles to adhere to the binder layer, and vaporizing the binder layer whereby to leave a thin, porous coating of the refractory particles on the strip stock during the final anneal.

This invention applies to the annealing of metals at temperatures which are so high with respect to the physical characteristics of the metals that sticking or welding would ordinarily occur in stacks of sheets or closely wound coils, unless the sheets or coil convolutions were separated by some refractory substance. For the sake of an exemplary embodiment, the invention will be described as applied to silicon-iron sheet stock containing about 3% silicon, and especially such sheet stock in which a predominant cubeon-face disposition of the grains or crystals is produced by surface energy secondary recrystallization in an appropriate heat treatment.

The term cube-on-face implies that two of the cube faces of the body-centered silicon-iron grains lie parallel, or substantially so, to the surfaces of the sheet stock, and is intended to be broad enough to include not only those products in which the cube edges essentially lack preferred orientation, but also those products in which the cube edges are predominantly oriented in directions parallel and perpendicular to the direction of rolling and perpendicular to the sheet surfaces. The last mentioned product have an orientation designatable as (100) [001] by Millers Indices, and are commonly referred to as having cubic texture.

The condition referred to above as cube-on-face is usually produced by reducing a suitable silicon-iron material to gauge in such fashion as will result (during a primary recrystallization) in the production of a number of grains or nuclei lying in or near the desired final position or orientation, and then causing these nuclei to grow at the expense of grains having other orientations in a secondary recrystallization at a higher temperature, which recrystallization takes place by surface energy. Such a secondary recrystallization is difficult to control, since it is affected by various conditions. For example, in the recently issued US. patent in the names of Kohler and Jackson, No. 3,130,095, dated Apr. 21, 1964, and entitled Production of Oriented Silicon-Iron Sheets by Secondary Recrystallization, it is taught that surface energy secondary recrystallization can be controlled and furthered by the presence of exceedingly minute quantities of polar compounds in the annealing atmosphere, e.g. less than about 250 ppm. of hydrogen sulfide. It was also taught in the US. patent of the present inventor, No. 3,090,711, issued May 21, 1963, that surface energy secondary recrystallization is facilitated by a very smooth condition of the stock being treated.

In the manufacture of silicon-iron stocks which are non- 3,389,006 Patented June 18, 1968 oriented, or which are to have a cube-on-edge orientation, the provision of an annealing separator is not as difficult. Such stocks are either not subjected to secondary recrystallization or, if they are, the secondary recrystallization occurs by grain boundary energy. In either event, the presence of a coating on the surfaces of the sheet stock does not interfere with the production of the final product.

Thus, it is possible to coat silicon-iron sheet stock which will ultimately have the cube-on-edge orientation with an adhesive slurry of magnesium hydroxide, and then dehydrate the coating by the application of heat, converting the magnesium hydroxide to magnesia. From the standpoint of adherence and refractoriness, this provides an excellent annealing separator. Further, if the silicon-iron sheet stock is subjected to conditions reducing to iron but oxidizing to silicon and carbon, silica formed on or near the surfaces of the stock can be fused at a high temperature with some of the magnesia, forming a glass coating which is useful for interlamination resistivity in magnetic cores. Such a method of applying or forming an annealing separator is not advantageous for cube-on-face silicon-iron, where the essential grain characteristics are developed in a secondary recrystallization proceeding by surface energy and the surface of the metal must be open to the annealing atmosphere.

There are various reasons for this, including changes in the condition of the applied coating material as, for example, when magnesium hydroxide is converted to magnesia, the difficulty of providing a smooth and even coating, the tendency under some circumstances to form a glass, and the fact that annealing separators applied and formed as described have a tendency to be non-porous. Also, water of hydration will form a continuous surface oxide which will inhibit surface energy grain growth.

It is a principal object of this invention to provide an annealing separator which will be free of these defects.

It is another object of the invention to provide a mode of imposing an annealing separator which not only can be carefully controlled as to thickness, porosity, and other characteristics, but which can be readily imposed and which will have sufiicient adhesion to the sheet stock to permit customary handling.

These and other objects of the invention, which will be set forth hereinafter or which will be apparent to the skilled worker in the art upon reading these specifications, are accomplished by that procedure and through the use of those materials of which exemplary embodiments will now be described.

As the features of primary importance in the practice of the invention, reference may be made to coating the surface of the sheet stock with a substance capable of acting as an adhesive, and of forming a layer of refractory substance in connectionwith the said adhesive material.

It may be stated at the outset that the adhesive material should be of dual character. It should comprise a vehicle which is vaporizable at comparatively low temperatures, i.e., well below the annealing temperature of the sheet stock, without reacting significantly with the silicon-iron or leaving an impervious residue. The second part of the adhesive is an organic binder substance dissolved in the vehicle which will also be relatively inert and substantially completely vaporizable at a higher temperature. This substance which will hereinafter be referred to as the binder substance, is the actual adhesive for the particles of annealing separator, the vehicle serving only to make it easier to apply the binder substance more uniformly and economically.

The vehicle may consist of various organic or inorganic substances, liquid at room temperatures, but almost completely vaporizable without substantial rise in temperature. The lower alcohols, ketones and similar substances may be used, although some organic vehicles may either produce bad working conditions or require expensive recovery equipment. Water forms an excellent vehicle when in combination with a suitable binder substance, since the vapor pressure of water is such that it may be dried in a thin coating either by blasts of air at normal or slightly elevated temperatures, or by a comparatively gentle degree of heat, and in any event without producing any change of undesired character in the surface of the metal sheet stock. It will be understood that a slight oxidation will later be reduced by the annealing treatment.

The binder substance to be dissolved or dispersed in the vehicle may take various forms. Its general characteristics have been given above. Where water is the vehicle, polyvinyl alcohol makes an excellent binder substance. It can be dissolved in the water to form a solution of a strength Varying from about /2% to about 5% but preferably about 2%. In any case, a minimum should be used, although greater amounts are applied when the coated material may be subjected to handling which will cause some of the coating to fall off. It will be understood that whereas most liquids are capable of forming a film which has certain binding characteristics before the liquid evaporates, the polyvinyl alcohol will have a binding effect after the evaporation of the water of solution. Yet, upon a further rise in temperature, the polyvinyl alcohol will itself vaporize without leaving an appreciable residue. The vaporization temperature of polyvinyl alcohol is about 500 F.

A film of water alone, or of the lower boiling organic liquids, is too evanescent to hold a layer of annealing separator in place during the normal handling operations involved in stacking sheets or winding coils. The use of liquid having greater viscosity and higher boiling point is not generally a solution to the problem for the reason that such liquids frequently dissociate during or prior to evaporation, and it is desirable to form an organic deposit on the sheet stock at high temperature or to produce at the surfaces of the stock a substance such as carbon which can be absorbed by the metal. Thus, for example, oil is not satisfactory because, upon decomposition, it increases the carbon content of the silicon-iron. The provision of a low-viscosity solution which is easily applied to the sheet stock, in which the vehicle or solvent is readily evaporable, but which contains a binder substance having a vaporization point higher than any temperature likely to be reached before the stock is stacked or wound for annealing, serves the purposes of this invention excellently providing both vehicle and binder substance are vaporizable without reacting with the metal or leaving an impervious residue on its surface.

Exemplary binder substance vehicle combinations are:

polyvinyl alcohol-water wheat pastewater urea formaldehyde-water latex-type adhesive-benzene latex-type adhesive-naphtha The above combinations have been used in concentrations up to 5% with good results.

Many latex adhesives will be satisfactory. Two latex adhesives which have been found to be satisfactory are EC-791 and EC-2204 marketed by the Minnesota Mining and Manufacturing Company. Latex performed well in naphtha and benzene vehicles. Other high volatile oil distillates could undoubtedly be used with latex or other binder substances.

There are various ways in which a refractory material may be associated with the vehicle-binder substance combination. It is possible to mix a finely divided non-hydrated and non-hydratable refractory substance with the binder solution to form a slurry, and then apply this in various ways to the surfaces of the sheet stock. The application may be done, for example, by spreading or doctoring or spraying. However, a slurry of the character referred to generally has substantial viscosity. When spread it may tend to coat the surface so as to form a barrier against surface energy reactions between the base metal and a gaseous annealing atmosphere. It is difficult to obtain an even coating by spraying; and under some circumstances a sprayed slurry may form separated spots of refractory thick enough to form dimples in a thin annealed product and thus affect space factor.

The preferred way of associating the refractory substance with the binder substance is to coat the sheet stock first with the vehicle and binder substance, and then afterwards deposit the refractory substance evenly onto the coated surface of the stock either before, during, or after the evaporation of the vehicle, but in any event while the layer is tacky.

The refractory separator substance should have a particle size between about 400 mesh and about mesh (Tyler Standard Screen Scale). The refractory is preferably one which will be unaffected in composition by temperatures up to and including the annealing temperatures. Thus it is disadvantageous to use magnesium hydroxide which could be dehydrated to magnesia during the heat treatment. Preferred refractory substances for use in the practice of this invention are alumina and non hydrating magnesia. Other inert refractory metal oxides may be used such, for example, as zirconia, titania and the like.

In the preferred practice of the process, the sheet stock at final gauge is first coated with the liquid binder composition. This may be done by means of coating rolls, sprays, doctors or the like; and precautions will be taken as by metering rolls or similar means to make the coating both thin and uniform. It will be understood by the workers in the art that the surfaces of the sheet stock must be in a condition to be wetted by the vehicle of the binder. Such a surface condition will generally exist unless the sheet stock surfaces have become covered with grease or oil, usually as a cold rolling lubricant. In this event the surfaces must be cleaned as by light pickling, alkaline cleaning, vapor degreasing, burning and the like.

When the surfaces of the sheet stock have been properly coated with the binder liquid, the stock is passed into a housing wherein actual deposition of the refractory separator substance occurs. Any method which will result in the deposition of a thin and uniform layer will serve. It is frequently possible to impose the powdered refractory substance by dusting it over the metal surfaces through screens. Methods of electrostatic deposition may be employed, and reference may here be made to US. Patent No. 3,000,752, dated Sept. 19, 1961. A preferred mode of deposition involves passing the adhesive coated sheet stock through the upper part of a vertically elongated housing, and fluidizing a quantity of the finely divided refractory in a known fashion within the housing by means of a current of air or other gas. In this Way either one or both sides of the base metal sheet or strip can be coated. There is some advantage in coating only one side because the uncoated side remains completely free to respond to conditions which promote secondary grain growth.

While it is preferable to apply the refractory to the adhesive in about the desired final thickness, the invention as herein described is broad enough to include the application of a greater quantity of the refractory substance and the subsequent removal of the excess by gravity or by a current of gas.

The adhesive liquid is applied to the sheet stock surfaces preferably in such quantity that a layer of binder substance less than about .005 inch will remain after the evaporation of the vehicle. The usual layer is less than .001 inch thick. This latter amount of binder substance will be found sufficient for the production of an annealing separator varying from about .002 to about .003 inch in thickness, which is generally satisfactory for the purpose, The actual thickness of the separator is of little consequence. The minimum is that necessary to prevent sticking of the sheets or convolutions during and after a high temperature treatment, while the maximum is dictated by economy and ease of handling.

During the passage of the sheet stock through the housing, it will be usual to drive off the vehicle by blasts of gas or gentle heat, or both, before assembling the sheet stock into a form suitable for annealing as by stacking or winding. It is advisable to drive off the vehicle to avoid the hazard of residual vehicle in the annealing separators at the time of the application of relatively high heat. The binder substance is small in total quantity and vaporizes with-out leaving a substantial residuum so that it may be eliminated during the anneal itself.

It will be understood that once a uniform layer of annealing separator is formed as described and the sheet stock assembled by stacking or by Winding into a coil, the refractory particles will be held in place by the juxtaposition of sheets or convolutions. Thus, the elimination of the binder substance by vaporization leaves the an nealing separator in a highly porous condition permitting interaction of the base metal and the annealing atmosphere; and the annealing separators of this invention have been found especially suitable for secondary recrystallization operations proceeding by surface energy.

The composition of the silicon-iron is not in itself a necessary limitation on the invention. However, a siliconiron' intended for cube-on-face orientation or for cubic texture will generally contain about 2.70% to about 3.30% silicon, 0.007% or less carbon in its final composition and from about 0.04% to about 0.12% manganese, the remainder being substantially all iron with such normal trace impurities as are incidental to the mode of manufacture, there being a total oxide content of 0.01% or less.

The initial sulfur content of the iron will he usually between 015% and 030%. The final product will have been dcsulfurized normally to less than 001% sulfur.

Example I Ladle analysis:

Si percent 2.72 C do .023 S do .021 Mn do .065 O p.p.m. 14

The processing steps were as follows:

(1) Hot roll to .1 in.

(2) Open anneal at'1675 F.

-(3) Pickle (4) Cold roll to .025 in.

(5) Decarburize strip to 008% carbon (3 min. at 1500 'F. in wet H (6) Coat with magnesia (7) Box anneal at 2200" F. for 30 hrs. in dry H (8) Pickle (9) Cold roll to .012 in.

( l0) Degrease (11) Coat with polyvinyl alcohol (4% Water solution) (12) Dust with alumina (13) Dry strip in furnace (about 15 sec. in 800 F. furnace) (14) Box anneal at 2200 F. for 36 hrs. (hydrogen sulfide in dry H The product consisted of substantially 100% cube texture grains.

Example II The same material as resulted from step 8 above was further processed as follows: (1) Cold roll to .0077 in. (2) Degrease (3) Box anneal at 1400 F. for 5 hrs. in dry H (4) Cold roll to .004 in. (5) Degrease (6) Coat with polyvinyl alcohol (1% water solution) (7) Dust with alumina 6 (8) Dry strip in furnace (5 sec. in 1000 F. furnace) (9) Box anneal at 2175 F. for 16 hrs, (hydrogen sulfide in dry H The product consisted of substantially cube texture grains.

Example III Ladle analysis:

Si percent 3.05 C do .022 S do .024 Mn do .092 O p.p.m. 30

Material of the above ladle analysis was processed as follows:

(1 Hot roll to .080 in.

(2) Open anneal at 1 675 F.

(3) Pickle (4) Cold roll to .026 in.

(5) Open anneal at 1675 F.

(6) Cold roll to .016 in.

(7) Decarburize strip to 003% carbon (3 min. at 1500 F. in Wet H (8) Coat with magnesia (9) 'Box anneal at 2200 F. for 30 hrs. in dry H (10) Pickle (11) Cold roll to .005 in.

(12) Degrease Box anneal at 1400 F. for 5 hrs. in dry H Cold roll to ,002 in.

Degrease Coat with polyvinyl alcohol (0.5% Water solution) Dust With alumina Dry strip in furnace (about 15 sec. in 800 F. furnace) (19) Box anneal at 2175" F. for 13 hrs. in dry H The product consisted of substantially 100% cube texture grains.

Modifications may be made in the invention without departing from the spirit of it.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for forming a refractory coating on sheet stock of silicon-iron prior to a final annealing operation which causes said stock to recrystallize into grains having predominantly a cube-on-face orientation, comprising the steps of:

(a) applying a thin binder layer in liquid form to at least one side of a clean strip of said stock, said binder layer comprising a binder substance in a vehicle, said vehicle having a vapor pressure lower than the vapor pressure of said binder substance, both said vehicle and said 'binder substance being capable of substantially complete vaporization at temperatures below said final annealing temperature;

(b) passing said strip and binder layer into contact with fine particles of a refractory separator substance to cause said particles to adhere to said binder substance, and

(c) vaporizing said binder layer whereby to leave only a thin, porous coating of said refractory particles on said strip stock during said final anneal.

2. The process claimed in claim 1 in which the said refractory material is in finely divided form having a mean particle size between about and 400 mesh.

3. The process claimed in claim 2 wherein the said binder layer is applied to the sheet stock in such quantity as to provide a coating of not more than about .005 inch thickness of the binder substance after evaporation of said vehicle.

4. The process claimed in claim 2 wherein the said binder layer is applied to the sheet stock in such quantity as to provide a coating of about .001 inch of the binder substance upon evaporation of said vehicle, and in which the total thickness of the said annealing separator is substantially .002 to .003 inch.

5. The process claimed in claim 2 wherein said vehicle is water and said binder substance is polyvinyl alcohol.

6. The process claimed in claim 1 including the steps of vaporizing said vehicle prior to said final anneal and vaporizing said binder substance during said final anneal.

7. A process for forming a refractory coating on sheet stock of silicon-iron prior to a final annealing operation which causes said stock to recrystallize into grains having predominantly a cube-on-face orientation, consisting of the steps of:

(a) applying a thin layer of binder substance in liquid form to at least one side of a clean strip of said stock, said binder layer being composed of materials capable of substantially complete vaporization at temperatures below said final annealing temperature.

(b) passing said strip and binder layer into contact References Cited UNITED STATES PATENTS 1,922,254 8/1933 -McCu1loch 117-22 X 2,641,556 6/1953 Robinson 1-17127 X 3,000,752 9/1961 Jackson et a1 117-169 X 6,090,711 5/1963 Kohler 148- 112 X 3,169,236 2/1965 McQuade 117-1 27 X 3,282,747 11/1966 Foster 117127 X WILLIAM D. MARTIN, Primary Examiner. 

